(a) Field of the Invention
The present invention relates to an optical system of endoscopes having flexible optical fiber and used for observing the inside of body cavities or inside of tubes in industrial field.
(B) Description of the Prior Art
For example, endoscopes now available for medical purposes for observing the inside of a stomach, esophagus, etc. can be classified into two typical types. One is the forward-viewing type for observing along the axis of the distal end of the endoscope and the other is the side-viewing type for observation in a direction at a right angle to the axis of the distal end. These two types have advantages and disadvantages respectively. For example, in a narrow tubular portion such as the esophagus, it is sometimes very difficult to use the side-viewing type endoscope because the sight for observation becomes narrow and, moreover, it is difficult to provide a sufficient optical length because the distance between the lens and object is short. By the forward-viewing type endoscope, it is possible to observe from comparatively distant positions to near positions. Therefore, in such narrow tubular portions, forward-viewing type endoscopes are generally used. On the other hand, in a wide baglike cavity such as a stomach, side-viewing type endoscopes are generally used because it is possible to observe all over the side walls. For such reasons, side-viewing endoscopes and forward-viewing type endoscopes are used for different applications according to characteristics of the portion to be observed so that their advantages can be fully utilized. When, however, it is required for example to observe the esophagus and stomach of one patient at the same time, it is necessary to use two endoscopes successively for the above reason. This is very inconvenient and, moreover, pain of the patient is extremely large. To solve the above problems, endoscopes which can be used for both of side-viewing and forward-viewing are provided. One example of such endoscopes is shown in FIG. 1. In this example, a mirror 1 is arranged at the end of an endoscope and it is arranged to carry out side-viewing as shown by the full line and forward-viewing as shown by the dotted line by inserting and removing the mirror 1. FIG. 2 shows another example of such endoscope in which it is arranged to continuously rotate the mirror 1. The former example, however, has a disadvantage that the image becomes upside down. That is, the image rotates. The latter example has advantages that the image does not become upside down and that the observing direction can be changed continuously. However, by the latter example, perfect forward-viewing is impossible and, moreover, the endoscope itself has to be made large.